Despite a year that has produced unprecedented ice melts in the Arctic and Greenland, a devastating drought across much of the country and hundreds of record high temperatures around the world, the subject of climate change has managed to remain in the deep freezer of presidential politics.

At a small rally in Miami Beach on Thursday, environmentalists took one last longshot at making global warming a meaningful campaign issue — hoping that if freaky global weather wasn’t enough, maybe salty Atlantic Ocean percolating up through the sewer grates of one of the state’s major tourist destinations might merit at least a question during the final presidential debate on Monday at Lynn University in Boca Raton.

“Nowhere else is the reality of climate change more visible than right here, right now on the corner of 10th and Alton in Miami Beach,” said Chrissy Elles, an outreach associate for the Union of Concerned Scientists, who stood barefoot in ankle deep water along with about 20 activists on the flooded sidewalk next to the Whole Foods parking lot. “This is an issue the next president of the United States will have to address.’’

The rally echoed a letter signed by 121 climate scientists, experts and political leaders in Florida, including mayors and commissioners from more than a dozen cities and counties, urging President Barack Obama and Republican challenger Mitt Romney to detail plans for dealing with a threat that will cost the state tens of billions of dollars in coming years.

Though climate change remains a subject of debate in some states, it’s already become a costly reality in South Florida. In Miami Beach, for instance, city commissioners are considering a $206 million overhaul of an antiquated drainage system increasingly compromised by rising ocean water.

Sea levels that have risen by eight inches over the last century have slowed the flow of runoff into the bay, producing prolonged flooding in low-lying streets after big storms. During extreme seasonal high tides, like ones this week, the drainage system does the reverse — conveying salty bay water onto streets. The city’s plan calls for more pumps, wells to store storm runoff, higher sea walls and “back-flow” preventers to block ocean and bay water from rising into streets.

The concerns aren’t confined to Miami Beach. Earlier this year, a report from Climate Central, an independent research and journalism organization, suggested Miami-Dade and Broward counties alone have more people vulnerable to flooding than any state except Florida and Louisiana. Other studies suggest some of the lowest-lying Florida Keys may be the first to be inundated.

Sam Van Leer, president of the Urban Paradise Guild, a Miami-based group focused on restoring native habitat, said the latest projections of accelerating sea rise level could overwhelm engineering fixes in coming decades unless steps are taken to reduce greenhouse gas emissions that fuel global warming.

“The water we’re seeing behind us is just the beginning,” he said. “It’s going to get worse.’’

Despite studies suggesting that the pace of warming is increasing, the issue is getting far less attention in 2012 than it did in 2008, when both Obama and his Republican challenger John McCain expressed concerns and endorsed efforts to reduce greenhouse gas. This time around, both campaigns have largely dodged the issue, focusing intensely on bread-and-butter concerns such as jobs, taxes and the sluggish economy.

Obama has given an occasional nod to the subject, promoting wind and solar power and mentioning climate change in his acceptance speech to the Democratic National Convention and during a campaign event last week in Miami, where he spoke about the threat of rising seas.

“That is not a joke. That is not a hoax. That’s our children’s future. Folks here in Miami understand that better than anybody,” Obama said. “The impact of climate change will be significant on our kids and grandkids unless we take those steps. We cannot just deny our way out of those things.”

For Republicans, skepticism about climate science, particularly the role humans have played in climate change, has become a litmus test. Romney has frequently mentioned his uncertainty. In his nomination acceptance speech, he used Obama’s lofty 2008 pledge “to slow the rise of the oceans and heal the planet” as a punch line, saying his goal was simply “to help you and your family.’’

Lisa Hoyos, co-founder of the Climate Parents, a recently formed California-based advocacy group, blames oil and energy companies for financing campaigns to attack climate science, efforts that have confused the public and given both parties political cover to back expanded oil drilling and coal use over alternative fuels.

Though Monday’s debate at Lynn is supposed to be focused on foreign policy, activists argue that the far-reaching implications of climate change, which could shrink food and water supplies and force massive population movements, deserve to be part of the discussion. With less than three weeks before the Nov. 6 election, it may be their last chance to put it on the national agenda.

Harold Wanless, a University of Miami geology professor who has documented decades of sea level rise in South Florida and signed the letter sent last week to both campaigns, said he’s frustrated by Washington politicians who took the issue seriously four years ago but now want to avoid it.

“I know we are worried about 10 million other things, but this is the one that is going dominate our next century,” he said.